Aurora 15 July 2012
I was enjoying a leisurely sweltering summer Sunday afternoon in the back yard with Evan, Sarah, two Adirondack chairs, a kiddie pool, and the schematics for an IC-290A I have on the bench. I came in to get a glass of water and while I was inside, I checked my e-mail (since I have some equipment for sale). No prospective buyers, but I did have a message from Sean, KX9X, that he was working aurora on 6 and 2 meters. I quickly plugged in the 2-meter rig and swung the beam around to the north. Sure enough, there were raspy aurora signals all over two meters. I quickly put N9GX (EN60) in the log for my first ever aurora QSO. This was at least as cool as working K5QE on 2-meter Es with 10 watts.
So, I fumbled around a drawer and pulled out a cable to connect the TS-700S to the computer and fired up Audacity. I made this interesting recording of KA1ZE/3. I started out with the beam to the NE (45 degrees azimuth) with a strong auroral buzz on Stan’s signal. Then I swung the beam around to the NNW direct path (345 degrees azimuth). I’m in FM19la and he’s in FN01xt, which is exactly 200 miles (322 km) direct path. On the direct (forward tropo scatter) path, there is still a hint of aurora, but the tone is a bit purer. When I turn the rotor there is pretty bad hum from a (not unexpected) ground loop.
In order to better visualize a few things, I ran a short-time Fourier transform (this is the actual technical term for a “waterfall”) on the audio file. I need to code-up a polyphase implementation of the FFT like that used in Rocky, but there are only so many hours in a day. Click on the image for full-size.
There are lots of interesting details here. First, you can see that the auroral scatter is both Doppler-shifted (lower in frequency) and Doppler-broadened (spread out from the central frequency) compared with the direct tropo scatter signal. Second, you can see the ground-loop-induced hum at the low-frequency end. Auroral backscatter comes from field-aligned plasma density irregularities embedded in the auroral convection flow. Because most readers will be allergic to the vector math, we make the (somewhat gross) approximation that KA1ZE and I are transmitting and receiving from the same location. Now, we can take a stab at estimating the flow velocity from the following equation:
Where delta-f is the Doppler shifted frequency (about 300-Hz from these data), c0 is the speed of light (300,000,000 m/s), f is the carrier frequency (144 MHz), and vflow is the flow velocity. While we’re making approximations, if we round f up to 150 MHz, the twos cancel and we get the Doppler shift of 300 Hz corresponding to a flow velocity of 300 m/s (670 mi/hr). Fast! Because it is lower in frequency than the direct signal, we can also infer that the flow was directed away from us.
There you have it! Science fair projects with your ham radio.
Love the analysis! Excellent article.